Corona discharge apparatus for particle collection

ABSTRACT

In corona discharge apparatus of the class including a source of high voltage and a corona discharge device energized therefrom, the source of high voltage is comprised by a source of alternating current and a piezoelectric transformer including a piezoelectric body, a pair of driving electrodes applied on the opposite surfaces of one end of the piezoelectric body and connected across the source of alternating current and an output electrode mounted on the other end of the piezoelectric body and connected to the corona discharge device and an electrostatic particle collector.

United States Patent H 1 Kawada Aug. 19, 1975 [54] CORONA DISCHARGE APPARATUS FOR 2,6l3 345 l0/l952 Osterland 323/93 UX 3.679.) l 8 7/l972 Keizi i 4 3 V8. l 1740926 (i/W73 Duval .i 55/139 X [75] Inventor: Takehiko Kawada, Yokohama.

Japan Primarv Examiner-J. D. Miller L l [73] Asblgnec ggll fgq g z d Assistant b \'ammer-Harry E Moose, Jr.

) p Armrney, Agent, or Firm-Charles E. Pfund Esq,

[22] Filed: Nov. 2, I973 Related US. Application Data Continuation of Ser. No, 228.638. Feb. 23 I972 abandoned.

[30] Foreign Application Priority Data Mar. 3. 197i Japan 4643943 Feb. 26 197i Japan 46-l l648 [52] U5. Cl. 317/3; 55/139; BIO/8.] [51] Int. Cl. HOlt 19/00 [58] Field of Search BIO/8.1; 321/15; 323/93; 55/l39 l45', 3l7/3, 4, 262 R [56] References Cited UNITED STATES PATENTS 2,469,23l 5/1949 Klemperer i. 55/l39 X 2 Claims, ll Drawing Figures Plllllll llll CORONA DISCHARGE APPARATUS FOR PARTICLE COLLECTION This is a continuation. of application Ser. No. 228,638 filed Feb. 23, I972, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to corona discharge apparatus especially suitable for use in electric dust collectors, ionizers, desmoking device or the like.

A conventional corona discharge device utilized in an electrophotographic machine, desmoking device, air cleaning device or the like for ionizing air or dust contained therein is generally constructed as shown in FIG. I, wherein the output voltage of an AC source 6 is stepped-up by a winding type step-up transformer I and the high voltage appearing across the secondary winding of transformer l is rectified by a diode 2. The DC output from the diode is applied across the electrodes of an ionizer 5 through a current limiting resistor 3 and an electric switch 4.

Ordinarily, the step'up transformer I has a drooping voltage current characteristic as shown by a curve shown in FIG. 2, whereas the ionizer or corona discharge dcvice has a voltage-current characteristic as shown by a curve I) shown in FIG. 2. When the voltage impressed across the electrodes of the ionizer is low, it does not ionize the air between the discharge elec trodes so that the output side of the high voltage source device is substantially in the open circuit condition. However, when the voltage impressed across the ionizer reaches a predetermined value. 8 KV for example, the air is suddenly ionized thus passing the load current. Once the load current begins to flow, ionization of the air is continuously carried out thus maintaining the terminal voltage across the ionizer at a substantially constant value.

Although this characteristic varies dependent upon the electrode spacing of the ionizer and variations in the ambient temperature and humidity, with the circuit arrangement shown in FIG. 1. since the internal resistance of the secondary winding of the step-up transformer is low, the load current varies greatly with the variation in the terminal voltage across ionizer 5.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide an improved corona discharge device whose load current varies only a little with the variation in the ter minal voltage.

Another object of this invention is to provide an improved high voltage source for the corona discharge device capable of withstanding short circuit conditions of the load.

Yet another object of this invention is to provide an improved high voltage source which generates a high voltage adapted to energize the corona discharge device and a medium voltage adapted to energize other loads.

According to this invention there is provided corona discharge apparatus of the class including a source of high voltage and a corona discharge device energized therefrom, characterized in that the source of high voltage comprises a source of alternating current and a piezoelectric transformer including a piezoelectric body, a pair of driving electrodes applied on the opposite surfaces of one end of the piezoelectric body and connected across the source of alternating current and an output electrode mounted on the other end of the piezoelectric body and connected to the corona discharge device.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. I shows a connection diagram of a prior art corona discharge apparatus;

FIG. 2 shows voltage-current characteristics of the step-up transformer and the corona discharge device shown in FIG. 1;

FIG. 3 shows a connection diagram of one example of the corona discharge apparatus embodying the invention;

FIG. 4 is a plot showing the output voltage output current characteristic of the piezoelectric transformer shown in FIG. 3;

FIG. 5 is a graph comparing the characteristics of the prior art corona discharge apparatus and the novel corona discharge apparatus;

FIGS. 6A, 6B and 6C show different examples of the corona discharge device;

FIG. 7 shows a connection diagram of a modified corona discharge apparatus constructed in accordance with this invention; and

FIGS. 8A and 8B show connection diagrams of portions of further modifications of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiment of this invention shown in FIG. 3 is suitable to operate an ionizer. According to this invention, the winding type step-up transformer shown in FIG. I is substituted by a piezoelectric transformer 21 including a bar shaped piezoelectric body 21d, a pair of driving electrodes 21a and 2112 applied on the opposite surfaces of one end of the piezoelectric body and an output electrode 21c applied on the opposite end surface of the piezoelectric body. When an AC voltage of a frequency equal to or substantially equal to the natural resonance frequency of the piezoelectric body 21d is impressed across the driving electrodes 21a and 211), a high AC voltage is created at the output electrode 2 It in a manner well known in the art. Since the internal resistance of the piezoelectric transformer 21 is high its output voltage-current characteristic varies as shown in FIG. 4 in which the ordinate represents the output voltage whereas the abscissa the output current. Referring again to FIG. 3 a rectifier circuit 22 including two diodes 23 and 24 is connected to the output electrode 2Ic of the piezoelectric transformer. The output terminal of rectifier circuit 22 is connected across the electrodes of corona discharge device 5 in series with a cur rent adjusting resistor 26. A voltage adjusting resistor 25 is connected in parallel with serially connected corona discharge device 5 and current adjusting resistor 26.

FIG. 5 compares the voltage-current characteristics of the corona discharge apparatus utilizing a conventional winding type step-up transformer and a piezoelectric transformer, in which the ordinate represents the output voltage and the abscissa the output current. Curve :1 represents the characteristic of ionizer 5, curves b and b the characteristics of the piezoelectric transformer and curves c and c the characteristics of a conventional winding type transformer. Characteristics b and c show those when the voltage is decreased percent than the case of b and c.

It will be noted that in case when the corona discharge device is operating at a point 1,, if the voltage decreases IO percent, with the conventional winding type transformer the operating point shifts to point 1 thereby greatly varying the load current (about 400 microamperes in the illustrated example). On the other hand, with the piezoelectric transformer, the operating point shifts to point 1;, causing smaller variation in the load current (about I80 microamperes).

In the case of the corona discharge apparatus utilizing an ordinary winding type step-up transformer as shown in FIG. 1, should the electrodes of the ionizer short eircuited due to accumulation of dust, the windings of the transformer would be burned out. Protective resistor 3 is connected in series with diode 2 for preventing a large short circuit current. On the other hand, according to this invention, since a piezoelectric transformer is used as the voltage step-up means, as the voltage-current characteristic of the piezoelectric transformer has a steep drooping characteristic as shown in FIG. 4, short circuit of the electrodes of the corona discharge device does not cause any dangerous short circuit current. For this reason. it is not necessary to use a large current limiting resistor causing a large power loss.

With this arrangement, when energized by the output voltage from AC source 6, the piezoelectric transformer 21 produces a high AC voltage at its output electrode 2]. This output voltage is rectified by a rectifier 22 and is then supplied to ionizer S, the voltage and current supplied thereto being adjusted by resistors 25 and 26, respectively.

In this embodiment, although rectifier 22 is used, it may be omitted if the AC output voltage from the piezoelectric transformer were sufficiently high to create corona discharge between the electrodes of ionizer 5.

Further, in this embodiment, the ionizer 5 comprises a grounded elongated electrode of semi-circular crosssection and a straight electrode extending at the center thereof, but it should be understood that the ionizer or corona discharge device many take various construe tions suitable for concentrating electric field.

Examples of the construction of the corona discharge device are shown in FIG. 6 wherein FIG. 6A shows a combination ofa grounded flat plate shaped electrode and a needle electrode 41 spaced apart therefrom a definite distance, FIG. 6B a combination of a grounded flate shaped electrode 42 and a plurality of needle electrodes 43 spaced apart therefrom a definite distance and are supported by an insulator 44 and FIG. 6C a combination of a grounded needle electrode 45 and another needle electrode 46 spaced apart from the needle electrode 45. The construction shown in FIG. 6B is especially suitable for use as a dust collector.

Resistor 25 shown in FIG. 3 can also be used as a protective resistance for piezoelectric transformer 21. More particularly, should the output electrode 210 be disconnected from the load while a large resonance voltage is being impressed across driving electrodes 21a and 21b of the piezoelectric transformer 21, the piezoelectric transformer would undergo free vibration and would be ruptured. Resistor 25 prevents such a no load condition. Resistor 25 may be substituted by a variable resistance element such as a Varistor. Then, its threshold characteristic is selected such that it is higher than the discharge voltage of the corona discharge device 5 but lower than the breakdown voltage of the piezoelectric transformer.

Another load may be used in combination with a corona discharge device. For example, in an electrostatic dust collector, fine particles in air or gas are electro statically charged by corona discharge and the charged particles are attracted by electric field. In such applications, it is necessary to provide a high voltage for the corona discharge electrodes and a medium voltage for electrodes for attracting the charged particles.

FIG. 7 shows a modified embodiment of this invention utilized as an electrostatic dust collector in which the same component elements as those shown in FIG. 3 are designated by the same reference numerals. In the embodiment shown in FIG. 7 there is provided a voltage multiplying rectifier circuit 7 comprising diodes 71 and 72, and capacitors 73 and 74 which are connected in a ladder-type network, in addition to diodes 23 and 24 shown in FIG. 3. Since the construction and operation of the voltage multiplying circuits are well known in that art, detailed description thereof is believed unnecessary. The rectifier circuit is provided with a high voltage terminal 76 connected to corona discharge device 5 and a medium voltage terminal connected to a collector 8 which attracts charged particles. This arrangement can supply to corona discharge device a stable energy of low current regulation. It can also supply a stable medium voltage to collector 8.

The illustrated voltage multiplying rectifier circuit 7 is composed of four diodes and two capacitors for multiplying the voltage by a factor of four, it should be understood that it is possible to increase the member of stages for producing higher voltages. Since it is possible to drive the load at lower impedance as viewed from the output terminal of the piezoelectric transformer, as the number of stages is increased it is possible to drive more efficiently the load which is required to operate with a constant current characteristic.

Although in this embodiment, the corona discharge device and the collector are energized by different voltages, it will be clear that they can be energized by the same voltage.

FIGS. 8A and 8B show modifications of the embodiment shown in FIG. 7 for assuring easy attraction of the charged particles, by shortening their path of movement while maintaining the electric field substantially constant. In these modifications dust collector 8 comprises three spaced apart parallel electrodes connected to terminals 81, 82 and 83 of which terminals 81 is con nected to the high voltage terminal 76, terminal 82 to a source of medium voltage and terminal 83 is grounded. More particularly, in FIG. 8A, terminal 82 is connected to the medium voltage terminal 77 (see FIG. 7) of voltage multiplying rectifier circuit 7 whereas in the case of FIG. 8B, terminal 82 is connected to the juncture between two capacitors 85 and 86 which are connected in series between the high voltage terminal of the voltage multiplying rectifier circuit 7 and the ground. Thus in either FIG. 8A or 8B the parallel electrode connected to terminal 82 will be at an intermediate potential and position between the parallel electrodes connected to terminals 81 and 83. The spacing between electrodes will be smaller (e. g.. onehalf the spacing between electrodes 81 and 83) and the electric field (voltage/distance ratio) will be constant. Thus the force on charged particles in the electric field will be constant but the average distance they must travel to reach an electrode will be one-half that required in FIG. 7. Consequently, the capacity of the collector 8 is doubled, the induced wind velocity is doubled or, alternatively, the size of the plates 81, 82, 83 can be reduced. While the invention has been shown and described in terms of certain preferred embodiments thereof it should be understood that many changes and modifications will be obvious to one skilled in the art without departing the true spirit and scope of the invention as defined in the appended claims.

What is claimed is:

I. An electrostatic precipitator comprising:

a source of alternating low voltage with one grounded terminal;

a high voltage ceramic piezoelectric transformer having input terminals connected across said low voltage and a high voltage. high impedance output terminal with no DC connection to ground;

rectifier means coupled to said output terminal includin g voltage multiplier means for producing relative to ground a high direct output voltage and an intermediate direct potential;

an atmospheric ionizing device having spaced electrodes one of which is grounded connected across said high direct output voltage and operative to electrostatically charge particles suspended in the medium between said electrodes; and a particle collector having spaced electrodes one of which is grounded connected across said high direct output voltage and a third electrode positioned between said spaced electrodes and connected to be maintained at said intermediate direct potential;

the grounded electrodes of said device and said collector being directly connected to said grounded terminal.

2. The electrostatic precipitator according to claim I wherein said particle collector comprises three spaced apart parallel electrodes, the outer electrodes of said parallel electrodes being connected across said high direct output voltage, a capacitor type voltage divider connected across said high direct output voltage. and the intermediate electrode of said parallel electrodes connected to an intermediate point of said capacitor type voltage divider. 

1. An electrostatic precipitator comprising: a source of alternating low voltage with one grounded terminal; a high voltage ceramic piezoelectric transformer having input terminals connected across said low voltage and a high voltage, high impedance output terminal with no DC connection to ground; rectifier means coupled to said output terminal including voltage multiplier means for producing relative to ground a high direct output voltage and an intermediate direct potential; an atmospheric ionizing device having spaced electrodes one of which is grounded connected across said high direct output voltage and Operative to electrostatically charge particles suspended in the medium between said electrodes; and a particle collector having spaced electrodes one of which is grounded connected across said high direct output voltage and a third electrode positioned between said spaced electrodes and connected to be maintained at said intermediate direct potential; the grounded electrodes of said device and said collector being directly connected to said grounded terminal.
 2. The electrostatic precipitator according to claim 1 wherein said particle collector comprises three spaced apart parallel electrodes, the outer electrodes of said parallel electrodes being connected across said high direct output voltage, a capacitor type voltage divider connected across said high direct output voltage, and the intermediate electrode of said parallel electrodes connected to an intermediate point of said capacitor type voltage divider. 